This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. P1058 Version 2.0 is an intensive 24-hour pK study of the pharmacokinetics of selected antiretroviral drugs in HIV-infected children >or <18 years of age who are receiving or about to initiate a regimen of antiretrovirals that includes one of the drugs or drug combinations of interest. The objectives are divided into three groups. 1) Assess the steady state by pK sampling of orally administered tenofovir given in combination with nucleoside reverse transcriptase inhibitors (NRTI) and non-nucleoside reverse transcriptase inhibitors (NNRTI) to older children and adolescents. 2) Assess the steady state by pK sampling of orally administered concomitant therapies such as boosted protease inhibitors (PI) such as darunavir/ritonavir used with tenofovir in older children and adolescents. 3) Describe the relationship between a subject?s body surface area (BSA) and tenofovir exposure during treatment with a fixed dosage at steady state with or without concomitant PI treatment.